Magnetron power supply and cathode heater circuit



July 9, 1968 w. c. HICKMAN 3,39

MAGNETRON POWER SUPPLY AND CATHODE HEATER CIRCUIT Filed May 24, 1965 INVENTOR.

WALLACE C. HICKMAN BY W A AGENT United States Patent 3,392,309 MAGNETRON POWER SUPPLY AND I CATHODE HEATER CIRCUIT Wallace C. Hickman, Wantagh, N.Y., assignor to North American Philips Co., Inc., New York, N.Y., a corporation of Delaware Filed May 24, 1965, Ser. No. 458,323 6 Claims. (Cl. 315-94) This invention relates to power supply circuits and more particularly to power supply circuits suitable for use with high power electron discharge devices of the magnetron type.

In the operation of a magnetron tube it is usually necessary that the cathode be heated to its proper operating temperature before the anode voltage is applied to the tube. If the anode voltage is applied at a time when only a small portion of the cathode surface has attained its operating temperature, a very large high density current will flow through the tube and since this current is concentrated at the small surface portion of the cathode, stress and damage to the cathode is brought about. A delay is therefore necessary between the application of the heater voltage and the anode voltage in order to provide time for the total cathode emitting surface to reach operating temperature so that the cathode is not subjected to the stress of a strong electric field during the warm-up period.

One common solution to the delay problem is to provide a separate heater and high voltage supply for the magnetron. It is then relatively easy to provide means for energizing the anode with a given time delay after the heater voltage is applied to the tube. This solution is expensive since it requires two separate transformers or other voltage supply sources. Another proposed solutionis to use a common transformer for the heater and high voltage supplies and to provide means in the high voltage portion of the circuit for delaying the application of the high voltage to the tube. This solution introduces the many problems associated with the switching of high voltages.

It is therefore an object of the present invention to provide an improved power supply system for a magnetron or other similar type high power electron discharge device which substantially eliminates the above described problems of the prior art systems.

It is another object of this invention to provide a magnetron power supply system utilizing a common voltage supply source for the heater and anode elements in which the attendant high voltage switching problems of the prior art systems are eliminated.

Another object of this invention is to provide a magnetron power supply circuit which uses a single transformer to supply operating voltages to the magnetron heater and anode elements and in which the delayed switching of the anode voltage occurs in the low voltage portion of the transformer circuit.

The above objects are obtained by utilizing a unique characteristic of the magnetron. The unique characteristic of the magnetron on which the invention is based is that a magnetron will draw an insignificant amount of anode-cathode current below the so-called anode voltage cut-oif point. The anode voltage cut-off point for most magnetrons is approximately 95 of its rated anode operating voltage. .In other words, if the magnetron anode voltage is below 95% of rated operating voltage, very little anode-cathode current will flow.

In accordance with the invention, a common source of energizing voltage is provided for both the heater and anode elements of the magnetron. The voltage source includes means for first applying a voltage to the anode and heater elements of the magnetron wherein the value of applied anode voltage is below the anode cut-off point of the magnetron. The heater voltage may also be slightly below the rated value for the magnetron, but it must be of a value sufficient to heat the cathode to its operating temperature. Automatic timing means are provided which apply the full rated anode and heater voltage to the magnetron after a time delay which is suflicient to bring the cathode to operating temperature. The magnetron is then in normal operation. In this manner the required high voltage delay can be obtained using a common transformer for the heater voltage and the anode high voltage and without the production of high density cathode currents.

In a preferred embodiment of the invention, a transformer is provided having a primary winding and first and second secondary windings which supply energizing voltages to the magnetron heater and anode elements, respectively. A resistor or other voltage dropping means is connected in series with the primary winding. The resistance value of the resistor is chosen so that during the Warm-up period the anode high voltage supply at the second secondary winding is limited to a value slightly below the anode voltage cut-off point. The heater supply voltage at the first secondary winding also will be slightly below its rated value, for example, approximately of rated value. However, this value of heater voltage is sufiicient to heat the cathode to its operating temperature. After the required delay, which is determined by a timing device, a relay or similar device is actuated to apply a short circuit across the voltage limiting resistor. The transformer then supplies the magnetron with the normal rated values of heater and anode supply voltages. The magnetron is then in normal operation.

An added advantage of the invention is that the series resistor can be designed to limit the heater current surge at the instant the supply voltage is turned on.

The foregoing and other objects and features of the invention will become apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawing wherein the single figure is a schematic diagram of a magnetron power supply system according to the invention.

Referring now to the drawing, there is shown a transformer 1 having a primary winding 2, a first secondary winding 3 for supplying heater or filament current to a magnetron 4 and a second step-up secondary winding 5 for supplying the high voltage for the magnetron anode structure 6. The ends of secondary winding 5 are connected to the input terminals of a full wave rectifier bridge circuit 7. The output terminals of the bridge circuit are connected across the anode-cathode structure of the magnetron and supply a full wave rectified voltage thereto. The microwave energy generated by the magnetron is supplied to a load by means of a coaxial cable 26.

An alternating current source of voltage, not shown, is applied across input terminals 8 and 9 of the circuit. One end of primary winding 2 is directly connected to terminal 9 and the other end is connected to terminal 8 by means of a path consisting of the parallel combination of limiting resistor 10 and relay contact 11 in series with a winding 12 of saturable reactor 13 and a switch 14.

One terminal of a timing motor 15 is directly connected to input terminal 9 and the other terminal is connected to input terminal 8 by means of switch 14 in series therewith. The field winding of motor 15 is not shown in the interest of clarity. The timing motor 15 operates a switch 16 by means of a mechanical coupling arrangement indicated generally at 17. The coupling arrangement 17 may include a gear train and cam, not shown, or other suitable drive mechanism for closing switch 16 after a predetermined number of revolutions of motor 15. There are numerous mechanical timers of this type which are commercially available and a detailed discussion thereof is not necessary for an understanding of the invention. Mechanical timers are available with both a fixed and an adjustable time delay. As an alternative, an electronic type of time delay which operates on the principle of charging a capacitor through a resistance can be substituted for the mechanical type of time delay described above. One possible circuit of this type comprises a DC operated relay serially connected with a variable resistor and a diode across a source of alternating current. A charge capacitor is connected in parallel with the relay coil and is charged from the AC source via the diode and variable resistor. The delay time can be adjusted by varying the R-C time constant of the capacitor charge circuit. When the capacitor charges to the relay pull-in voltage, the relay is energized and closes suitable contacts.

Relay coil 18 is connected across the input terminals 8 and 9 via a series circuit including switches 14 and 16. Relay contact 11 is normally open in the dcenergized condition of the relay. After a suitable time delay sufiicient to heat up the magnetron cathode, relay 18 is energized and contact 11 is closed, thereby short-circuiting voltage limiting resistor and effectively removing same from the circuit.

In order to regulate and control the operating voltages applied to the magnetron, a voltage regulating circuit 19 is connected between the input terminals 8, 9 and primary Winding 2.

The regulation circuit includes a saturable reactor 13 having a so-called gate Winding 12 serially connected in one supply line to primary winding 2. Reactor 13 also includes a feedback winding 20 serially connected with an adjustable resistor 21 between one terminal of winding 12 and one input terminal of a control rectifier bridge circuit 22. The other input terminal of bridge circuit 22 is directly connected to input terminal 9 by means of conductor 25. Reactor 13 further includes a DC control winding 23 which is serially connected with a resistor 24 across the output terminals of bridge circuit 22.

The operation of the saturable reactor regulation circuit is not important to an understanding of the invention and will therefore only be outlined briefly. The saturable reactor circuit maintains the voltage supplied to the primary winding of transformer 1 reasonably constant by sensing changes in the line voltage and current applied to primary winding 2 through winding 12 of the saturable reactor and inverting this voltage change by means of the feedback winding. For example, if the line voltage at terminals 8, 9 and' the line current through winding 12 increases, then the feedback voltage occurring between the output of the feedback winding 20 and the conductor 25 decreases. This results in a lower voltage being applied to the control rectifier bridge circuit 22, and as a consequence the direct current suplied to the DC control winding 23 of the saturable reactor is reduced. This in turn reduces the saturation of the reactor core which eifectively increases the reactance of the gate Winding 12. The increased impedance of winding 12 produces a larger voltage drop across this winding which counteracts the initial increase in line voltage and tends to maintain a constant voltage between the output of the gate winding and terminal 9. Resistor 21 is adjustable so as to set the operating current of the magnetron to a predetermined value. The purpose of resistor 24 is to improve the time-constant of the control circuit so as to reduce the response time of the circuit. If desired, a resistor, not shown, may be connected in parallel with the gate winding 12 in order to improve the linearity of the regulation curve.

In the deenergized condition of the circuit, switches 14 and 16 and relay contact 11 are all open. When switch 14 is first closed, timing motor and primary winding 2 are energized. Switch 16 is open at this time and so relay 18 remains unenergized and contact 11 is open. As. a result, series limiting resistor 10 reduces the applied voltage to primary winding 2 to a value such that the induced voltage in secondary winding 5 is below the anode cut-off voltage of magnetron 4. This prevents magnetron 4 from drawing anode current until the cathode has been heated to its operating temperature. The value of resistor 10 may be chosen to produce approximately of rated voltage in the heater winding 3 and the high voltage winding 5 during the warm-up period. After the required time delay, which is determined by timing motor 15 and the mechanical coupling arrangement 17, switch 16 is closed, relay coil 18 is energized, and contact 11 is closed. Limiting resistor 10 is then effectively removed from the circuit and the full rated heater and anode voltages are aplied to magnetron 4. It will be readily apparent that there will be little or no arcing across contact 11 since a relatively low voltage is being switched at that point in the circuit. The high voltage switching problems of the prior art devices have been substantially eliminated by means of the invention.

Although the invention has been described with reference to a particular embodiment thereof, it is to be understood that the invention is not limited to the particular details of construction described, as many equivalents will suggest themselves to those skilled in the art. Therefore, it is to be clearly understood that the invention is not to be limited to the specific arrangement shown and described except as defined by the appended claims.

What is claimed is:

1. A power supply circuit for an electron discharge device of the type having a cathode and an anode and being characterized by the feature that the anode current is substantially zero below a given value of anode voltage, comprising a transformer having an input winding and a first high voltage supply winding and a second supply winding, means connecting said first winding across the anode and cathode of said device, means connecting said second winding to said device so as to heat said cathode, and a source of alternating voltage connected to said input winding and comprising means for selectively applying a first alternating voltage to said input winding and thereafter a second alternating voltage having a value greater than the first voltage, said first voltage having a value sufficient to heat said cathode and insufficient to cause an appreciable flow of anode current in said device.

2. A power supply circuit for a magnetron of the type having a cathode and an anode, comprising a transformer having an input winding and a first high voltage supply winding and a second supply winding, means connecting said first windingacross the anode and cathode of said magnetron, means connecting said second winding to said magnetron so as to heat said cathode, a pair of input terminals for a source of alternating voltage, and variable voltage means serially connected with said input terminals and said input winding for varying the voltage suppliedto said magnetron, said variable voltage means comprising means for selectively applying anoutput voltage having first and second values to said input winding, said first voltage value producing in said first winding a voltage which is less than the anode cut-off voltage for said magnetron and said second voltage value producing in said first winding a voltage which is at least equal to said anode cut-off voltage.

3. A power supply circuit for a magnetron of the type having a cathode and an anode, comprising a transformer having an input windingand a first high voltage supply winding and a second supply winding, means connecting said first winding across the anode and cathode of said magnetron, means connecting said second winding to said magnetron so as to heat said cathode, a pair of input terminals for a source of alternating voltage, variable voltage means energized from said input terminals for supplying first and second levels of voltage to said input winding, said first voltage level producing in said second winding a voltage which is sufiicient to heat said cathode approximately to operating temperature and producing in said first winding a voltage which is less than the anode cut-off voltage for said magnetron, said second voltage level producing in said first winding a voltage which is greater than said anode cut-01f voltage whereby anode current flows in said magnetron, and timing means energized from said input terminals for controlling said variable voltage means to change from said first voltage level to said second voltage level after a predetermined period of time sufiicient to heat said cathode to operating temperature.

4. A magnetron energizing circuit comprising a mag netron device having an anode and a cathode, a transformer having an input winding and a first high voltage supply winding and a second heater supply winding, means connecting said first winding across the anode and cathode of said device, means connecting said second winding to said device so as to heat said cathode, a pair of input terminals for a source of alternating voltage of a given magnitude, a resistor serially connected with said input terminals and said input winding and having a value of resistance sutlicient to reduce the voltage applied to said input Winding to a first voltage level which is sufficient to heat said cathode approximately to normal operating temperature but which is less than the anode cut-off voltage for said magnetron, switch means connected across said resistor, and timing means energized from said voltage source for actuating said switch means after a given time interval to short circuit said resistor thereby to apply to said input winding a second higher voltage which produces in said first winding a voltage which is greater than said anode cut-off voltage whereby anode current flows in said magnetron.

5-. A magnetron energizing circuit comprising a magnetron having an anode, a cathode and a heater, a transformer having a primary winding and first and second secondary windings, means connecting said first winding across the anode and cathode of said magnetron, means connecting said second winding to said heater, a pair of input terminals for a source of alternating voltage, a switch, a resistor, means connecting said switch, said resistor, said input terminals and said primary winding in a first series circuit, a timing motor having a contact, means connecting said switch, said timing motor and said input terminals in a second series circuit, a relay coil having a normally open contact connected in parallel with said resistor, means connecting said switch, said motor contact, said relay coil and said input terminals in a third series circuit, said first series circuit supplying a first operating voltage to said primary winding upon closure of said switch of a magnitude sutficient to heat said cathode to operating temperature by means of said second winding but which produces in said first winding a voltage which is less than the anode cut-oil voltage for said magnetron, said timing motor being responsive after a given time interval sufficient to heat said cathode to close said motor contact and thereby energize said relay coil via said third series circuit, said relay contact simultaneously closing to short-circuit said resistor and thereby apply a second value of operating voltage to said primary winding which produces in said first winding a voltage which is at least equal to the anode cut-off voltage for said magnetron whereby anode current flows in said magnetron.

6. A magnetron energizing circuit comprising a magnetron device having an anode and a cathode, a transformer having an input winding and a first high voltage supply winding and a second heater supply winding, means connecting said first winding across the anode and cathode of said device, means connecting said second winding to said device so as to heat said cathode, a pair of input terminals for a source of alternating voltage of a given magnitude, variable impedance means serially connected between said input terminals and said input winding for varying the voltage applied thereto, timing means energized from said input terminals for controlling said impedance means between first and second impedance levels, said first impedance level producing at said input winding a voltage having a first value which is sutficient to produce in said second winding a voltage which is sufiicient to heat said cathode to operating temperature and producing in said first winding a voltage which is less than the anode cut-01f voltage for said magnetron, said second impedance level producing at said input winding a voltage having a second value which produces in said first winding a voltage which is greater than said anode cut-0E voltage whereby anode current flows in said magnetron.

References Cited UNITED STATES PATENTS 5/1957 Kozikowski 315-102 X 11/1967 Hurlimann 315-102 

1. A POWER SUPPLY CIRCUIT FOR AN ELECTRON DISCHARGE DEVICE OF THE TYPE HAVING A CATHODE AND AN ANODE AND BEING CHARACTERIZED BY THE FEATURE THAT THE ANODE CURRENT IS SUBSTANTIALLY ZERO BELOW A GIVEN VALUE OF ANODE VOLTAGE, COMPRISING A TRANSFORMER HAVING AN INPUT WINDING AND A FIRST HIGH VOLTAGE SUPPLY WINDING AND A SECOND SUPPLY WINDING, MEANS CONNECTING SAID FIRST WINDING ACROSS THED ANODE AND CATHODE OF SAID DEVICE, MEANS CONNECTING SAID SECOND WINDING TO SAID DEVICE SO AS TO HEAT SAID CATODE, AND A SOURCE OF ALTERNATING VOLTAGE CONNECTED TO SAID INPUT WINDING AND COMPRISING MEANS FOR SELECTIVELY APPLYING A FIRST ALTERNATING VOLTAGE TO SAID INPUT WINDING AND THEREAFTER A SECOND ALTERNATING VOLTAGE HAVING A VALUE GREATER THAN THE FIRST VOLTAGE, SAID FIRST VOLTAGE HAVING A VALUE SUFFICIENT TO HEAT SAID CATHODE AND INSUFFICIENT TO CAUSE AN APPRECIABLE FLOW OF ANODE CURRENT IN SAID DEVICE. 